Suppose one has $1,000,000 in a suitcase and they are flying to the US. Suppose they check the box in their customs form that says they're bringing more than $10,000 with them. They show the form to the CBP agent. What happens next?
The US Customs and Border Protection website states that there is no limit to the amount of currency that can be brought into or taken out of the US.
There is no limit on the amount of money that can be taken out of or brought into the United States. However, if a person or persons traveling together and filing a joint declaration (CBP Form 6059-B) have $10,000 or more in currency or negotiable monetary instruments, they must fill out a "Report of International Transportation of Currency and Monetary Instruments" FinCEN 105 (former CF 4790).
The CBP site also notes that failure to declare currency and monetary instruments in excess of $10,000 may result in its seizure.
Further, the site states that the requirement to report currency on a FinCEN 105 does not apply to imports of gold bullion.
However, the legal website The Law Dictionary includes details of how money laundering laws may come into play here :
As part of the War on Terror and the War on Drugs, U.S. law enforcement agencies have significantly increased their vigilance over money laundering. To this effect, travelers who carry large amounts of cash without supporting documentation of its legitimate source may be subject to secondary inspections and seizure of funds. In some cases, law enforcement may confiscate cash in excess of $10,000 until supporting documents are produced.
So far, I have described the "official" position. However, reading between the lines, I think it is fair to say that in the current climate if you show up at an entry point with a suitcase full of a large amount of cash you would face considerable scrutiny, regardless of any supporting documentation you may present.
If you fail to present supporting documentation, then I think your cash would certainly be seized. If you are a US resident, then you would be given the opportunity to obtain satisfactory documentation. If you did present documentation, then I think your cash would be held for as long as it would take to verify the validity of the documentation. Failure to present valid documentation would result in money laundering charges being brought against you and the matter would rest before the courts.
If you are not a US resident, then failing to produce supporting documentation would mean your cash being seized and entry into the US would almost certainly be denied. You would then have to deal with the situation from outside of the US. If you did produce supporting documentation, then again I suspect the cash would be held for as long as it takes to verify the validity of the documentation. Whether or not you were allowed to enter the US would depend on what other documentation you possess.
Since all the other answers thus far seem to downplay the risk (likelihood) of the money being seized, I figure I may as well make my comment an answer.
Unless you happen to have your legal team travelling with you and your suitcase of cash, you should expect that you'll be questioned extensively, so that any sign of nervousness, inconsistency in your answers or anything you say that doesn't "make sense" to the officer will be used as an excuse to seize your money, and you'll learn an expensive lesson in civil asset forfeiture.
The government will file a complaint against your money, leading to a ridiculously named case, such as United States v. $124,700 in U.S. Currency. Worth noting that while the outcome in this case was not in the government's favor, in the vast majority of cases, the government keeps the cash. Between 9/11 and 2014, U.S police forces have seized over 2.5 billion dollars in cash without search warrants or indictments and returned the money in less than 10% of cases. That last link is kind of a long read, but contains cases where people with completely legitimate money and documentation for their money had it seized anyway, and were only able to recover it after months or years in court.
The $10,000 mark is not a ceiling in importing cash, but rather a point where an additional declaration needs to be made (Customs Form 4790). At 1 million, I suspect you might be in for a bit of an interview and delay.
Here's an explanation of what happens when the declaration isn't made: https://www.cbp.gov/newsroom/local-media-release/failure-declare-results-seizure-24000-arrest-two
Once you declare the amount, the CBP officials will ask you the source and purpose of funds.
You must be able to demonstrate that the source of funds is legitimate and not the proceeds of crime and it is not for the purposes of financing terrorism.
Once they have determined that the source and purpose is legitimate, they will take you to a private room where two officers will count and validate the amount (as it is a large amount); and then return the currency to you.
For nominal amounts they count it at the CBP officer's inspection desk.
Once they have done that, you are free to go on your way.
The rule (for the US) is any currency or monetary instrument that is above the equivalent of 10,000 USD. So this will also apply if you are carrying a combination of GBP, EUR and USD that totals to more than $10,000.
Bad plan. This seems like a recipe for having your money taken away from you by CBP. Let me explain the biases which make it so.
US banking is reliable enough for the common citizen, that everyone simply uses banks. To elaborate, Americans who are unbanked either can't produce simple identity paperwork; or they got an account but then got blacklisted for overdrawing it. These are problems of the poor, not millionaires. Outside of determined "off the grid" folks with political reasons to not be in the banking and credit systsm, anyone with money uses the banking system. Who's not a criminal, anyway.
We also have strong laws against money laundering: turning cash (of questionable origin) into "sanitized" cash on deposit in a bank. The most obvious trick is deposit $5000/day for 200 days. Nope, that's Structuring: yeah, we have a word for that.
A guy with $1 million cash, it is presumed he has no choice: he can't convert it into a bank deposit, as in this problem - note where she says she can't launder it.
If it's normal for people in your country to haul around cash, due to a defective banking system, you're not the only one with that problem, and nearby there'll be a country with a good banking system who understands your situation. Deposit it there. Then retain a US lawyer who specializes in this, and follow his advice about moving the money to the US via funds transfer. Even then, you may have some explaining to do; but far less than with cash.
(And keep in mind for those politically motivated off-the-financial-grid types, they're a bit crazy but definitely not stupid, live a cash life everyday, and know the law better than anybody. They would definitely consider using banks and funds transfers for the border crossing proper, because of Customs. Then they'll turn it into cash domestically and close the accounts.)